1. Field of the Invention
The present invention relates to a light receiving module and a light receiving method with reduced polarization dependence, and particularly to a light receiving module and a light receiving method with reduced polarization dependence used in evaluation of polarization characteristics of various optical parts.
2. Description of the Related Art
It has been desired to reduce the amount of change (hereinafter called xe2x80x9cpolarization dependencexe2x80x9d) in light output detected by a light receiving element in a light receiving module used in evaluation of polarization characteristics of various optical parts. A related art for reducing this polarization dependence is disclosed in, for example, JP-A-7-318766. The related art will be described in detail below.
FIG. 3 is a schematic configuration diagram of a light receiving module in the related art.
In FIG. 3, incident light 21a emitted from an optical fiber 21 is focused through a first lens 22 and passes through an light transmission plate 24 and is focused through a second lens 26 and is received by a light receiving element 23. This light receiving element 23 includes a glass plate 23a and a light receiving surface 23b. 
Incidentally, an evaluation apparatus (not shown) of polarization characteristics measures a light output of the incident light 21a emitted from the optical fiber 21 to evaluate the polarization characteristics by analyzing an electrical signal into which the incident light 21a is converted by the light receiving element 23.
The light receiving module in this related art will be described in further detail.
In FIG. 3, the first lens 22, the second lens 26, and the light receiving surface 23b of the light receiving element 23 are inclined at a several angle with respect to an optical axis of the incident light 21a to receive the incident light 21a, respectively.
In this case, when light reflected by the first lens 22, the second lens 26 and the light receiving surface 23b returns to the side of a light source 27, for example, in laser light, characteristics of the side of the light source 27 deteriorate, so that a precise light outout cannot be measured.
Next, even in case that the light output of the incident light 21a does not change, when the light output is measured with high precision (requiring a precision of the order of 3/1000 dB), a polarization state of the incident light 21a or a change in wavelength cannot be ignored.
Due to inclinations of the first lens 22, the second lens 26 and the light receiving element 23 with respect to the optical axis of the incident light 21a, a difference in the quantity of transmission light depending on a polarization state of the incident light 21a occurs in the light receiving surface 23b of the light receiving element 23 in which a difference in a refractive index occurs.
Incidentally, the quantity of transmission light is quantity of light obtained by subtracting quantity of reflected light from total quantity of light of the incident light 21a and is the quantity of light detected in the light receiving element.
Also, even in case that the light output is constant incident light 21a, there is a problem (polarization dependence) in that the light output finally detected in the light receiving element 23 varies with a change in the polarization state (for example, a change from linear polarization to circular polarization).
In order to solve the polarization dependence or each of the problems, in this related art, the light transmission plate 24 is placed adjacent to the light receiving surface 23b of the light receiving element 23 and is inclined with respect to the optical axis of the incident light 21a and is inclined at such an angle that polarization dependence due to the inclination of the light receiving surface 23b of the light receiving element 23 has the same value as polarization dependence occurring due to the inclination of the light transmission plate 24, and an inclined direction of the light transmission plate 24 is held at a 90xc2x0 twisted position relation with respect to an inclined direction of the light receiving surface 23b of the light receiving element 23.
However, there is an influence due to variations during assembly such as an angle deviation of lens holding parts (not shown) used in the light receiving module or an inclination angle deviation in adjusting the optical axis, so that there is a problem in that the polarization dependence cannot fully be canceled even when using the related art described above.
The invention is implemented in view of the problem, and an object of the invention is to provide a light receiving method and a light receiving module for reducing polarization dependence occurring in the case of inclining a light receiving element at an arbitrary angle with respect to an optical axis of incident light and for being resistant to influence of a change in a polarization state of the incident light.
A light receiving module with reduced polarization dependence according to the invention can solve the problem by the following description and achieve the object of the invention.
1. There is provided a light receiving module with reduced polarization dependence comprising:
a first lens;
a second lens;
a first light transmission plate disposed between the first lens and the second lens;
a second light transmission plate disposed between the first lens and the first light transmission plate; and
a light receiving element having a light receiving surface, the light receiving element disposed adjacent to the first light transmission plate with the second lens sandwiched;
wherein the light receiving element is disposed so that the light receiving surface is not perpendicular to an optical axis of an incident light beam;
the first optical transmission plate is disposed so that a polarization dependence of the first optical transmission plate is equal to a polarization dependence of the light receiving surface, and the first optical transmission plate is rotated by 90xc2x0 with respect to the optical axis; and
the second optical transmission plate is disposed so that the sum of a polarization dependence of the second optical transmission plate and the polarization dependence of the first transmission plate is equal to the polarization dependence of the light receiving surface to cancel out the remaining polarization dependence, and the second optical transmission plate is rotated by 90xc2x0 with respect to the optical axis.
2. At least one of the first light transmission plate and the second light transmission plate is a parallel plane plate.
3. At least one of the first light transmission plate and the second light transmission plate is a wedge plate.
Also, a light receiving method with reduced polarization dependence according to the invention can solve the problem by the following description and achieve the object of the invention.
4. A light receiving method with reduced polarization dependence, comprising the steps of:
disposing a first lens;
disposing a second lens;
disposing a first light transmission plate between the first lens and the second lens;
disposing a second light transmission plate between the first lens and the first light transmission plate;
disposing a light receiving element having a light receiving surface, adjacent to the first light transmission plate with the second lens sandwiched so that the light receiving surface is not perpendicular to an optical axis of an incident light beam;
inclining the first optical transmission plate so that a polarization dependence of the first optical transmission plate is equal to a polarization dependence of the light receiving surface;
rotating the first optical transmission plate by 90xc2x0 with respect to the optical axis;
inclining the second optical transmission plate so that the sum of a polarization dependence of the second optical transmission plate and the polarization dependence of the first transmission plate is equal to the polarization dependence of the light receiving surface to cancel out the remaining polarization dependence; and
rotating the second optical transmission plate by 90xc2x0 with respect to the optical axis.
5. At least one of the first light transmission plate and the second light transmission plate is a parallel plane plate.
6. At least one of the first light transmission plate and the second light transmission plate is a wedge plate.